[Javascript] What is JavaScript Function Currying?

Currying is a core concept of functional programming and a useful tool for any developer's toolbelt.

Example 1:

let f = a => b => c => a+b+c;

let result = f(1)(2)(3);
console.log(result); // 6

Example 2:

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JS Bin</title>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.18.0/ramda.min.js"></script>
</head>
<body>
<div id="one" style="width: 100px;height: 100px;border: 2px solid black">One</div>
<div id="two" style="width: 100px;height: 100px;border: 2px solid black">Two</div>
<div id="three" style="width: 100px;height: 100px;border: 2px solid black">Three</div>
</body>
</html>

let one = document.getElementById('one');
let tow = document.getElementById('two');
let three = document.getElementById('three');

let f = a => b => c => a.addEventListener(b, (event)=>{
  event.target.style.background = c;
});

f(one)('click')('red');

Example 3:

let one = document.getElementById('one');
let tow = document.getElementById('two');
let three = document.getElementById('three');

let f = a => b => c => a.addEventListener(b, (event)=>{
  event.target.style.background = c;
});

let oneClickEvent = f(one);
let twoClickEvent = f(two);


oneClickEvent('mouseover')('red');
twoClickEvent('mouseout')('blue');

Example 4: include ramda library:

let one = document.getElementById('one');
let tow = document.getElementById('two');
let three = document.getElementById('three');

let f = R.curry((a,b,c) =>{
  a.addEventListener(b, (event)=>{
    event.target.style.background = c;
  });
});

// with placeholder from Ramda
const clickGreen = f(R.__, 'click', 'green');
clickGreen(one);

Example 5: multi placeholders:

let one = document.getElementById('one');
let tow = document.getElementById('two');
let three = document.getElementById('three');

let f = R.curry((a,b,c) =>{
  a.addEventListener(b, (event)=>{
    event.target.style.background = c;
  });
});

// with placeholder from Ramda
const clickColor = f(R.__, 'click', R.__);
clickColor(one, 'yellow');
clickColor(two, 'grey');
clickColor(three, 'pink');